Conventionally, a rotor is known to have a core body including a through-hole. For example, JP-2012-143090A (corresponding to US 2012/0169163A1) discloses a rotor having a core body which includes five through-holes arranged in a circumference direction of the core body at a regular interval. By having the five through-holes, the rotor may be lighter in weight and may have less inertia.
Generally, when a core body has a through-hole, a rotor may have reduction in rotation balance, magnetic balance, and strength. According to the rotor disclosed in JP-2012-143090A, the core body has a spoke portion. The spoke portion and the through-hole are alternately arranged in the circumference direction. By having the spoke portion, the strength of the core body may be restricted from decreasing. Further, by contriving shapes of the through-hole, the magnetic balance may be restricted from reducing. For example, the through-hole has a trapezoid shape in cross-section, in other words, an outer wall surface of the through-hole in a radial direction of the core body has a flat plane shape (see FIGS. 19-23 in JP-2012-143090A). However, the rotation balance may not be improved while the magnetic balance is improved.
JP-2012-143090A also discloses a rotor having a sector-shaped through-hole, in other words, an outer wall surface of the through-hole in the radial direction has a curved shape (see FIG. 18 in JP-2012-143090A). However, a specific shape such as curvature radius is not described as to the outer wall surface of the through-hole and a polar portion of the core body.